1. Field of the Invention
The field of the invention is that of surface acoustic wave transducers and filters including such transducers, used in numerous fields such as mobile radio communication systems for example for intermediate frequency filtering.
2. Discussion of the Background
Various types of high-performance transducers have come out over the last ten years or so.
Unidirectional transducers (Single-Phase Unidirectional Transducers also known as SPUDT) have replaced bidirectional transducers in many applications by virtue of the decrease in losses which they make it possible to obtain. This type of transducer, described in the published U.S. Pat. No. 2,702,899 is made by insetting, into a transducer, so-called transduction cells and so-called reflection cells, and by positioning the cells with respect to one another in such a way that the waves emitted are again in phase with the waves reflected in the useful direction and that phase opposition is obtained in the other direction. This involves a transducer in which electrodes designed so as to achieve the existence of a transduction function and a reflection function are dispersed. It has also been demonstrated in the published U.S. Pat. No. 2,702,899 that it may be advantageous to make resonant cavities inside the SPUDT, a resonant cavity being made by changing the sign of the reflection function.
For the customary substrates, the distance between transduction centre and reflection centre can be of the form (2n+1)xcex/8 with n an integer so that the phases are correct.
However, the quality factor Q relating to the ratio of the capacitance to the conductance of the filter and representative of the bandwidth and insertion loss of the filter is lower for unidirectional filters with their specific architectures into which are introduced asymmetries than that of conventional, symmetric bidirectional filters.
To increase performance, that is to say to increase the coupling in a unidirectional transducer (without increasing the capacitance), the invention proposes a transducer into which is introduced an etching network superimposed on the conventional networks of electrodes of acoustic transducers. More precisely, the subject of the invention is a surface wave transducer including a substrate on which are deposited two networks of interdigital electrodes and connected to different polarities so as to create acoustic transduction cells defined by at least two consecutive electrodes of different polarities, characterized in that it furthermore comprises at least one network of etchings separated by mesas, the superposition of the networks of electrodes and of the etching network making it possible to obtain a favoured direction of propagation of the acoustic waves.
According to a first variant of the invention, the networks of electrodes are symmetric with respect to an axis situated at the centre of two consecutive electrodes of the same polarity, the network of etchings being asymmetric with respect to the said axis.
In this configuration, the symmetric networks of electrodes make it possible to retain a high coupling factor whilst for its part the asymmetric etching network makes it possible to create the favoured direction of propagation of the acoustic waves.
Advantageously, the surface wave transducer may comprise a succession of at least two etching networks which are asymmetric so as to locally reverse the favoured direction of propagation of the surface waves.
According to another variant of the invention, the networks of electrodes define sets of 3 electrodes of different width per characteristic wavelength corresponding to the central frequency of operation of the transducer in which sets, a first electrode and a second electrode are separated by a distance 3xcex/16, and are connected to different polarities, the second electrode and a third electrode being separated by a distance xcex/8, in such a way as to define a favoured direction of propagation of the acoustic waves, the electrodes being positioned on the mesas of the etching network.
The advantage of such a configuration resides in the increase in the reflection coefficients of the electrodes. In conventional structures, to increase these coefficients, it is necessary to increase the thickness of the electrodes; by using a structure of mesas separated by etchings, the electrodes being deposited on the surface of the mesas, the coefficient of reflection of the electrodes is thereby increased whilst maintaining a small thickness of metallization.